Clean, lightly sand, vacuum off dust, prime, paint. Would help if you have us more information. What are you painting, condition it in, etc.?
What are you trying to paint?
Depends on what you want to paint, but always start with a clean surface. If it's furniture, wipe it down and remove any residue or dirt. If it's walls, at least dust them. Use quality brushes and rollers for best results.
A small bathroom and it’s original paint from the builder si think i will need to prime first, but what is a good paint for this kind of surface?
Unless you’re going from dark to light, have a tricky color to cover or the walls are not in good shape, you don’t need to prime. Any quality latex paint will be just fine. A paint with a primer in it, as most paints are these days, will do the job.
Do not be tempted to settle for the low price paint. Cheaper paint does not cover as well, drips more ( thus more wastage) and does not have the adhesiveness or durability that higher-end brand names do.
Using mildew-resistant paint in bathrooms is recommended.